1. Technical Field
The present invention relates to an electrographic image forming apparatus such as a printer, a copier, a facsimile and the like.
2. Background Art
In general, the electrographic image forming apparatus includes a fixing portion for fixing a toner image on a sheet of paper. Such a fixing portion has a heating roller having any heating source built-in, an upper pressuring roller, an endless fixing belt that stretches the heating roller and the upper pressuring roller, and a lower pressuring roller that is arranged to face the upper pressuring roller (Heating Belt System).
In the image forming apparatus, when performing a fixing process, the upper and lower pressuring rollers are clamped to form a fixing nip portion. This nip portion allows a toner image to be fixed on the sheet of paper on which an image forming portion has transferred the toner image, by passing the sheet of paper through the nip portion to be heated and pressurized. In addition, although the heating belt system has been described in the above description, the fixing portion can adopt a heating roller system in which the upper pressurizing roller has a heating source built-in and the upper pressuring roller itself is formed as a heating portion.
By the way, during the fixing process of the fixing portion, a toner-image-supporting portion of the sheet of paper, in which the toner image has been not yet fixed, directly contacts the fixing belt and the upper pressurizing roller (hereinafter, a case where the fixing belt is used will be described.) In this moment, wax oozed out of the toner image adheres to a side of fixing belt so that a latent image by the wax can be formed on the side of fixing belt. In such a condition, when a next sheet of paper is conveyed, the wax may be transferred to the image formed on this sheet of paper. Specifically, when fixing the toner forming the image on the next sheet of paper, the wax adhered to the fixing belt or the like is moved to the next sheet of paper. This enables to appear on the next sheet of paper a phenomenon in which gloss unevenness may appear on a portion in the next sheet of paper to which a large amount of wax adhere and a portion in the next sheet of paper to which a small amount of wax adheres (hereinafter, referred to as “gloss memory”). This gloss memory can cause image quality to be deteriorated.
Accordingly, in order to prevent the image quality from being deteriorated by the gloss memory, it is a known technology that the roller and the sheet of paper are slipped by generating any braking force to a direction of obstructing the rotation of the upper or lower pressuring roller to inhibit the wax from adhering to the side of fixing belt. For example, the following examples as the technology for applying the braking force to the roller constituting the fixing portion will be given.
Japanese Patent Application Publication No. 2007-183682 discloses a fixing mechanism which has a slide bearing for applying a braking force to control a speed-up phenomenon of the rotation before a rear end of the sheet of paper is passed through the fixing nip portion of the heating roll for fixing.
Japanese Patent Application Publication No. 2006-256708 discloses an image forming apparatus in which a brake roller applies a predetermined rotation load to the pressuring roller to slow down a rotation speed of the pressuring roller so that a load for allowing the speed of the sheet of paper which is apart from the heating roller in the double sheets of paper to slow down can be applied.
Japanese Patent Application Publication No. 2006-58758 discloses an image forming apparatus which has a braking member utilizing electroviscous fluid in which its viscosity varies based on a load variation to control a braking force of this braking member based on the load variation.